Generically, both hot melt adhesives and thermosetting urethane adhesives are well known generic adhesive classes. Compositions known as hot melt adhesives have the advantage that they can be conveniently applied by extruding the compositions at elevated temperature directly onto a substrate and can result in the formation of a structural bond with another substrate as the temperature of the adhesive mass cools. While hot melt adhesives have many benefits they have the drawback that the compositions tend to be temperature sensitive. In other words the bonding mass tends to lose bond strength as the temperature of the bonding mass and the substrate is increased. Further, hot melt adhesives can be physically unstable in the presence of hydrocarbon solvents and other organics.
In sharp contrast, while thermosetting moisture-cure urethane adhesives commonly have little initial green strength, such thermosetting compositions can cure over time to form a rigid, resilient, highly crosslinked bond of high tensile strength. Such adhesives have value where initial strength is not important since substantial bonding often forms over a period of time between a matter of hours to days. Typically such urethane adhesives are used by clamping the substrate-adhesive-substrate combination together with a sufficient amount of the adhesive and permitting the adhesive to cure over a period of time in order for the adhesive to form a fully set bond. However, thermosetting adhesives can have weak green (uncured) bonds. Further, many polyurethane adhesives have a short useful life, less than 10 hours, when maintained at elevated temperature during storage and use. Clearly the combination of moisture cure and hot melt properties in a single adhesive is a desirable goal since the resulting adhesive, in theory, could possess a quick high strength green bond and a strong crosslinked adhesive joint.
One suggested adhesive that combines hot melt properties and thermosetting moisture-cure urethane properties is disclosed in Uchigaki, U.S. Pat. No. 3,931,077, which teaches a reactive hot-melt adhesive composition comprising a reactive urethane prepolymer, a thermoplastic polymer, and a tackifying resin. Uchigaki suggests that hot-melt moisture cure urethane thermosetting adhesives should have a urethane prepolymer having a viscosity higher than 300,000 cP at 25.degree. C. Uchigaki suggests that viscosities substantially lower than that disclosed in Uchigaki result in adhesive compositions having suitable viscosity for application but having inferior instant adhesive strength (green strength). Further, Uchigaki teaches that tackifier components that can be used in the hot-melt-thermosetting adhesives should be a terpene phenol copolymer or abietic acid type resins whose active hydrogens or double bonds are at least partly removed by esterification. Such compositions include tackifier resins such as a hydrogenated rosin, a hydrogenated rosin glycerine ester, a hydrogenated rosin pentaerythritol ester, disproportionated rosin, polymerized rosin, etc. Uchigaki further teaches that other types of tackifiers are reactive with isocyanate groups on the prepolymer composition and are therefore undesirable. Further, Uchigaki suggests that still other types of tackifiers are insufficiently miscible with the adhesive components to result in a stable blend.
Uchigaki suggests using thermoplastic polymer compositions in the adhesive composition such as an ethylene vinyl acetate copolymer. Uchigaki teaches that the ethylene component of the copolymer is in the range of 70-90 wt-% of the total polymeric composition. Having an ethylene component less than 70 wt-% in the final adhesive composition results in poor final adhesive strength.